Numerical investigation of heat transfer characteristics in utility boilers of oxy-coal combustion

Applied Energy xxx (2014) xxx–xxx

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Numerical investigation of heat transfer characteristics in utility boilers of oxy-coal combustion Yukun Hu a,⇑, Hailong Li b, Jinyue Yan a,b a b

Royal Institute of Technology (KTH), Department of Chemical Engineering and Technology/Energy Processes, SE-100 44 Stockholm, Sweden Mälardalen University, School of Sustainable Development of Society and Technology, SE-721 23 Västerås, Sweden

h i g h l i g h t s  Air-coal and oxy-coal combustion in an industrial scale PF boiler were simulated in ANSYS FLUENT.  The O2 concentration of 33 vol% in the oxy-coal combustion case matches the air-coal combustion case most closely.  The moisture in the flue gas has little impact on flame temperature, but positive impact on surface incident radiation.

a r t i c l e

i n f o

Article history: Received 20 October 2013 Received in revised form 28 February 2014 Accepted 20 March 2014 Available online xxxx Keywords: Oxy-coal combustion Boiler Radiation Heat transfer Wet recycle

a b s t r a c t Oxy-coal combustion has different flue gas composition from the conventional air-coal combustion. The different composition further results in different properties, such as the absorption coefficient, emissivity, and density, which can directly affect the heat transfer in both radiation and convection zones of utility boilers. This paper numerically studied a utility boiler of oxy-coal combustion and compares with air-coal combustion in terms of flame profile and heat transferred through boiler side walls in order to understand the effects of different operating conditions on oxy-coal boiler retrofitting and design. Based on the results, it was found that around 33 vol% of effective O2 concentration ([O2]effective) the highest flame temperature and total heat transferred through boiler side walls in the oxy-coal combustion case match to those in the air-coal combustion case most; therefore, the 33 vol% of [O2]effective could result in the minimal change for the oxy-coal combustion retrofitting of the existing boiler. In addition, the increase of the moisture content in the flue gas has little impact on the flame temperature, but results in a higher surface incident radiation on boiler side walls. The area of heat exchangers in the boiler was also investigated regarding retrofitting. If boiler operates under a higher [O2]effective, to rebalance the load of each heat exchanger in the boiler, the feed water temperature after economizer can be reduced or part of superheating surfaces can be moved into the radiation zone to replace part of the evaporators. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Carbon dioxide (CO2) has been considered as one of the most important greenhouse gases. A substantial part of the emission comes from fossil fuel based power generation systems [1]. As coal has much higher carbon intensity than oil or natural gas, large scale pulverized coal power plants may have the highest potential for the application of CO2 capture technologies. Oxy-fuel combustion is a leading attractive CO2 capture technology, which has the potential to retrofit conventional coal fired steam power plants. Many efforts have been concentrated ⇑ Corresponding author. Current address: University of South Wales, Llantwit Road, Pontypridd CF37 1DL, United Kingdom. Tel.: +44 1443 483445. E-mail address: [email protected] (Y. Hu).

to improve this technology, such as Li et al. [2,3] studied the impurities and flue gas purification in oxy-fuel combustion process; Hu et al. [4,5] investigated the characterization of flue gas in oxy-coal combustion processes; Stadler et al. [6] researched oxy-coal combustion by efficient integration of oxygen transport membranes; Álvarez et al. [7,8] numerically inspected oxy-coal combustion in an entrained flow reactor. However, currently the oxy-fuel combustion technology has been only investigated experimentally in lab scale and pilot scale combustion units, for example the 0.5 MW combustion test facility [9] and Schwarze Pumpe 30 MW pilot plant [10]. Modeling of combustion processes by computational fluid dynamics (CFD) has become state-of-the-art for conventional air combustion [11]. Nevertheless, the application of CFD modeling for oxy-fuel combustion needs adaptation since the radiative heat transfer is altered due to the different

http://dx.doi.org/10.1016/j.apenergy.2014.03.038 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.

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Nomenclature Ar, s1 or kg/m2 s Pa pre-exponential factor D0, m2/s diffusion rate coefficient Er, J/kmol activation energy V0, kg/m3 theoretical air/oxidant amount Vdomain, m3 volume of fluid domain a, – stoichiometric coefficients of volatile b, – stoichiometric coefficients of char

Abbreviation CFD computational fluid dynamics EDM eddy dissipation model FGR flue gas recycle RANS Reynolds-Averaged Navier–Stokes equations RTE radiative transfer equation UDF user-defined-function WSGG weighted sum of gray gases

Symmetry Plane

Symbols Adomain, m2 area of fluid domain

E C

F A-INFERIOR ECONOMISER B-SUPERIOR ECONOMISER C-PRIMARY SUPERHEATER

BURNERS 11430×15000

E-FINAL SUPERHEATER F-REHEATER

A

FURNACE

2600 11000

7000

35000

B

11000

Over-fire ports

D-INTERMEDIARY SUPERHEATER

2600

47000

D

Z Y

X

Burners

3600

DIMENSIONS IN mm

3×2600

3600

Boundary air ports

15000

Fig. 1. Geometry of the utility boiler [18].

combustion gas atmosphere [12–14]. In addition, knowledge gaps still exist in the integration of oxy-fuel combustion technology in the large-scale power systems [15]. Therefore, the current research efforts are also focused on how to apply oxy-fuel combustion technology in a full boiler [16]. Our previous work [17] numerically studied a combustion test facility, which capacity of 0.5 MW. The models of coal devolatilization, volatile combustion, char burnout, and radiation absorption coefficient etc. were built in FLUENT with the consideration of oxy-coal combustion conditions. The CFD model has been validated by the experiment on the combustion test facility [9]. The numerical simulation results showed that the effective oxygen (O2) concentration1 ([O2]effective) of ca. 30 vol% in the oxy-coal combustion, which corresponds to the flue gas recycle ratio of ca. 71%, results in the most similar flame temperature and radiation heat transfer to the air-coal combustion. However, it still remains uncertain whether it is the same result for a large scale boiler, since the emissivity of radiating gases is a function of the domain-based beam length (or geometry dimensions) of combustion units. Therefore, this work was conducted in order to investigate the flame profile and the heat transferred through walls in both large scale

air-coal combustion boiler and large scale oxy-coal combustion boiler. In particular, considering the possible change of the heat loads of economizer, evaporator, and superheater when retrofitting an oxycoal boiler, the effects of different operating conditions, e.g. higher [O2]effective and wet recycle option (see details below) on oxy-coal boiler retrofitting were further analyzed and discussed based on the results calculated from the numerical simulations. The findings of this work will provide guideline of operation conditions to design oxy-coal combustion boilers, like oxygen concentration and flue gas recycle options. 2. Description of the utility boiler Numerical simulations of pulverized coal combustion were performed regarding a front wall fired boiler with an installed capacity of 300 MW [18]. The utility boiler is 47 m high, 15 m wide, 11.43 m deep. Twenty burners are arranged in a 4  5 matrix (four burners disposed in each level). The detail dimensions are shown in Fig. 1 and the wall boundary conditions are listed in Table 1. 3. Modeling approach

1 The effective O2 concentration is defined as the average O2 concentration in the primary and secondary air streams.

The commercial software, ANSYS FLUENT 13.0 [19], was used to build the model to simulate combustion, fluid and particle flow, as

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3.2. Geometry discretization

Table 1 The specifications of wall boundary condition [18]. Wall

Side wall

Furnace bottom

Furnace exit

Furnace hopper

Superheater

Emissivity, – Temperature, K

0.6 620

1 350

1 1000

1 350

0.6 620

well as heat and mass transfer inside the utility boiler. When the coal particles travel through the boiler, drying, devolatilization, volatile combustion, and char burnout occur in series according to specific models. The radiative transfer equation (RTE) was solved by using the discrete ordinate model [20]. The weightedsum-of-gray-gas (WSGG) model [21] was used in FLUENT via a user-defined-function (UDF) to calculate absorption coefficient. The discrete phase (coal particles) was modeled by using the Eulerian–Lagrangian approach with pressure-based solver to this three-dimensional problem. The SIMPLE algorithm was used for velocity–pressure coupling, and the realizable k–e turbulence model was considered for Reynolds-Averaged Navier–Stokes (RANS) equations. The boundary condition of constant wall temperature was applied for all walls of the boiler.

The mesh system of fluid domain was built according to the detail dimensions, which consisted of 800,000 hexahedral cells taking into account the accuracy and computing time. The mesh was refined at the location of the burners, where the ignition and most of the combustion reactions occurs, as shown in Fig. 2. 3.3. Assumptions and input data Currently, the oxy-coal combustion mainly refers to the O2/CO2 recycle combustion. The recycled CO2-rich flue gas replaces N2 in the combustion. Therefore how to recycle the flue gas will directly affect the thermal performance of the boiler. In order to remove

3.1. Key numerical models 3.1.1. Coal devolatilization model The competing reaction model that incorporates the influence of the heating rate was applied to predict the volatile yield in devolatilization. It assumes that two reactions proceed simultaneously and compete for hydrogen molecules. One reaction dominates at low temperatures and the other at high temperatures [22]. The detailed kinetic data for the coal devolatilization in the simulation are given in Table 2.

CoalðsÞ ! aI VolatileI ðgÞ þ bI CharI ðsÞ þ . . . CoalðsÞ ! aII Volatile2 ðgÞ þ bII CharII ðsÞ þ . . .

ðIÞ ðIIÞ

3.1.2. Volatile combustion model (volumetric reactions) Two-step reaction mechanism (reactions III and IV) governed by eddy dissipation model (EDM) was considered in this study, which has an acceptable accuracy in the prediction of CO2 and H2O concentrations compared with four-step reaction mechanism, but lower time-intense in the calculation [24].

Fig. 2. Grid used for the simulation.

Volatile þ 1:436O2 ! 1:313CO þ 2:036H2 O þ 0:067N2 þ 0:0096SO2 ðIIIÞ CO þ 0:5O2 ! CO2

ðIVÞ

3.1.3. Char burnout model (surface reactions) The kinetics/diffusion-limited combustion model was used to describe the heterogeneous reactions on char surface. Both diffusion and intrinsic kinetics were considered in this model, in which the surface reaction rate is determined either by kinetics or by a diffusion rate. The related kinetic data for char burnout in the simulation are diffusion rate coefficient (D0, 5  1012 m2/s), pre-exponential factor (Ar, 0.0059 kg/m2 s Pa), and activation energy (Er, 6.248  107 J/kmol) [25]. Table 2 Reaction kinetic data of coal devolatilization [23]. Reaction

ai

Ar,i, (s1)

Er,i, (J/kmol)

I II

Volatile 1

3.7  105 1.46  1013

7.366  107 2.511  108

Fig. 3. Swirl burners’ arrangements.

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coal moisture at a relatively low temperature, the recycled flue gas acting as primary air must be dry. Secondary air does not have this constrain, and it is more flexible. So there are two potential options for the secondary air. One is dry recycle option, in which the flue gas is recycled after condensing; the other is wet recycle option, in which the flue gas is recycled before condensing. However, these two options would affect the heat transfer inside the radiation zone due to the change of radiating gas content in the flue gas. More information about how to calculate the flue gas recycle rate/ratio can refer to our previous studies [4,5]. In addition, the platen superheater was not considered as the actual tube geometry and in the model, it was simplified as a heat sink source with internal emissivity of 0.6 at temperature of 620 K. Other heat

exchangers, like reheater and economiser, were not considered in this work. The actual air/oxidant amount was calculated as the theoretical air/oxidant amount plus excess air/oxygen in order to control the exit O2 concentration to be 3.5 vol%. The theoretical air/oxidant amount was estimated by Eq. (1), which was originally derived to calculate theoretical air amount in air-coal combustion [26]. In the air-coal combustion (Case I), the [O2]effective is 21 vol%; and in the oxy-coal combustion, the [O2]effective is changed from 29 vol% to 37 vol% according to the specific cases.

V0 ¼

1 ð0:01867C ar þ 0:05567Har þ 0:007Sar ½O2 effective  0:007Oar Þ ðkg=m3 Þ

Table 3 Analysis of coal used in the simulation [9]. Value Proximate (as received) Volatile matter content, % Ash content, % Fixed carbon content, % Moisture content, %

33.55 11.98 48.27 6.20

Ultimate (as received) Car, % Har, % Oar, % Nar, % Sar, % Gross heating value, MJ/kg

65.91 4.59 8.89 2.09 0.34 27.10

Table 4 Comparison of calculated results and experiment data.

Furnace exit temperature, K Furnace exit O2 concentration, vol% Unburned carbon, % Total heat to walls, MW Heat to superheaters, MW

Experiment data

Calculated results

1287 3.5 3.8 330 63

1398 3.4 4.5 297 66

(Y=8.9 m) Ref. case [O2]effective= 21 vol%

ð1Þ

Due to the lack of detail geometrical dimensions of input ports, such as burners, boundary air and over-fire air ports, the dimension of each port was estimated based on the flow rate (m3/s) and velocity (m/s) of the corresponding stream, because each specific stream of boiler has a typical range of velocity and temperature to assure the smooth transportation and stable combustion of pulverized coal. We referred a utility boiler manual [26], and assumed the typical velocities and temperatures for these specific streams. Specifically, 30% of the total oxidant transporting pulverized coal enters from the internal annulus of the burner as primary air at 25 m/s and 453 K; 50% of the total oxidant enters from the external annulus of the burner as secondary air with a swirl number of 0.6 and at 35 m/s and 623 K, arranged as Fig. 3. In addition, 15% of the total oxidant is supplemented directly through over-fire ports at 50 m/s and 623 K; the remaining oxidant is considered as boundary air, and enters from boundary ports at 15 m/s and 453 K. Therefore, one air-coal combustion case (Reference case) and four oxy-coal combustion cases are included in this study: three case with dry recycle option and different [O2]effective (Case I: 29 vol%; Case II: 33 vol%; Case III: 37 vol%) and one case with wet recycle option and 33 vol% of [O2]effective (Case IV). All cases were simulated based on the same input rate of coal 29.4 kg/s as [18]. The properties of the coal are listed in Table 3, which are used to set the turbulence chemistry mechanisms and the particle injections. The mean diameter of coal particle is

Case I 29 vol%

Case II 33 vol%

Case III 37 vol%

Fig. 4. The predicted temperature contours for air- and oxy-coal combustion conditions.

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56.3 lm, and the dispersion of particle size (spread parameter) is 1.4 [17].

3.4. Comparison of calculated results with experiment data Due to no available experiment data about large scale (more than 300 MW) oxy-coal boilers currently, the CFD model built in this study cannot be validated directly. However, the experiment data for large scale air-coal boilers is available [18]. Table 4 shows the comparison of calculated results and experiment data in terms of specific operating parameters. The calculated results are comparable with the experiment data in all operating parameters except the furnace exit temperature. The difference might be due to the different coal gross heating value used in the simulation. The coal gross heating value used in this study is 27.098 MJ/kg. However, the coal gross heating value was not given in Ref. [18]. As we known, a large coal-fueled electrical generating plant peaks at about 46% of efficiency [27]. Assuming the reference power plant

5

operates at this efficiency, the coal gross heating value is about 22.15 MJ/kg. Therefore, the difference can be considered as acceptable. In view of the above results, the current models and algorithms used for the simulations are acceptable. In addition, as mentioned before same models of the key components such as coal devolatilization, volatile combustion, and char burnout; properties such as the radiation absorption coefficient; procedures have been validated by the available experiment data from the combustion test facility, as reported in our previous publication [17]. These models and algorithms can be further used for the simulation of oxy-coal combustion in large scale boilers.

4. Results 4.1. Effects of [O2]effective (based on dry recycle) [O2]effective is an important parameter in oxy-coal combustion. It affects not only the flue gas recycle ratio and/or mass flow rate [16]

Fig. 5. The predicted surface incident radiation contours for air- and oxy-coal combustion conditions (From left to right: front wall, left wall, rear wall, right wall).

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directly, but also the flame profile inside the boiler and the total heat transferred through walls. 4.1.1. Flame profile The predicted temperature contours under air-coal and oxycoal combustion conditions are illustrated in Fig. 4. As expected, the flame temperature rises with the increase of the [O2]effective of oxidant. For the highest flame temperature reached in each case, it is 2200 K in Case II, which matches the reference case most closely. Besides the different values of the highest flame temperature in the studied case, the flame shape in those cases is also different from each other slightly. meanwhile, it is worth noting that the oxy-coal combustion always results in a flame temperature about 400 K higher than the air-coal combustion in the region close to the burners. This is due to the lower momentum of gasflow in the oxy-coal combustion cases compared with the air-coal combustion case. Since the high production costs of O2, the O2 concentration in the flue has to be limited as lower as possible (3 vol% in this study). The oxidant amount in each stream is reduced with the increasing of [O2]effective [refers to Eq. (1)]. Therefore, although each stream has the same velocity, their momentums are lowered. it is consistent with our previous study on the 0.5 MW combustion test facility [17]. In addition, the reduced oxidant amount could further affect the ignition process and combustion stability. From the heat balance point of view, the less amount of oxidant, the less heat is required to heat pulverized coal stream (coal particles + oxidant) to the ignition point and therefore the faster ignition. Coal particles have a longer residence time for complete combustion. However, too little amount of oxidant (means that the momentum is too small) is not conducive to achieve a stable combustion. When the velocity of oxidant streams is slower than the velocity of flame propagation, the flame could reversely propagate and cause the damage of burners. 4.1.2. Surface incident radiation The predicted surface incident radiation contours for air-coal and oxy-coal combustions are illustrated in Fig. 5(a–d). Similar to the flame temperature, the surface incident radiation also rises with the increase of the [O2]effective of oxidant. However, although the flame profile of Case II matches the reference case most closely, its surface incident radiation does not match as well as the flame profile. The surface incident radiation is weaker on the side and rear walls but stronger on the front wall. This is due to that the flame penetration is weakened by the decrease of the gasflow

Fig. 6. The predicted temperature contours for dry and wet recycle options in oxycoal combustion.

momentum in the oxy-coal combustion case. It further results in the flame core contracting and moving forward to the burners. Table 5 lists the calculated heat flux (kW/m2) on the specific walls, including both radiation and convection heat transfer. Although the amounts of the total heat transferred through walls are similar in the reference case and Case II, the two cases have different radiation heat transfer on the specific walls. Compared to the reference case, 8.6% more heat and 5.4% less heat were transferred through the front and rear walls in case II by the radiation. Moreover, about 90% of the total heat is transferred by radiation in the boiler radiation zone, which is consistent with others’ results [18]. And this ratio rises with the increase of the [O2]effective as well. 4.2. Comparison of dry and wet flue gas recycle In order to identify the effect of moisture on flame profile, the predicted temperature contours for dry and wet recycle options under the oxy-coal combustion condition were calculated and illustrated in Fig. 6. It can be seen from the figure that the increase of moisture content in the oxidant stream has little impact on the flame temperature. This is mainly due to the small change of the flue gas heat capacity, because CO2 and H2O account for more than

Table 5 The calculated heat flux on the specific walls. Reference case [O2]effective = 21 vol% Convection heat transfer, MW Front wall Left wall Right wall Rear wall Radiation heat transfer, MW Front wall Left wall Right wall Rear wall Total heat transfer, MW Front wall Left wall Right wall Rear wall Summation, MW

Case I 29 vol%

Case II 33 vol%

Case III 37 vol%

Case IV 33 vol%(wet)

7.46 5.56 5.61 8.06

9.74 6.84 6.73 10.06

9.75 6.48 6.48 9.80

9.81 6.53 6.74 10.30

9.48 6.27 6.30 9.39

63.23 61.43 61.38 84.48

60.59 54.88 54.60 72.64

68.65 60.75 60.28 79.92

76.94 65.79 65.68 86.06

71.70 62.63 62.41 83.35

70.69 66.99 66.99 92.54 297.21

70.33 61.72 61.33 82.70 276.08

78.40 67.23 66.76 89.72 302.11

86.75 72.32 72.42 96.36 327.85

81.18 68.90 68.71 92.74 311.53

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This can be explained by the fact that the emissivity of H2O is almost two times of the emissivity of CO2 [29]. It reveals that the increase of moisture content in the flue gas is an effective way to improve the radiation heat transfer, whereas the increased moisture content may further lead to the rise of flue gas dew point temperature [30]. Therefore, moisture content in the flue gas has to be considered and designed especially when applying the wet recycle option to an oxy-coal boiler. 5. Discussions 5.1. Scale effects on oxy-coal combustion

Fig. 7. The predicted contours of O2 molar concentration for dry and wet recycle options in oxy-coal combustion.

95% of the flue gas in the two cases, 85 mol% CO2 + 12 mol% H2O in dry case and 80 mol% CO2 + 17 mol% H2O in wet case, and the difference of heat capacities of CO2 (1.4 kJ/kg K at 2000 K) and H2O are (2.8 kJ/kg K at 2000 K) not too much [28]. Thus the weighted heat capacities (1.526 kJ/kg K in dry case and 1.596 kJ/kg K in wet case) are close. In addition, similar results were found for the predicted contours of O2 molar concentration. As shown in Fig. 7, the O2 of the air injected into the furnace was quickly consumed during the combustion reactions, because the temperature inside the furnace was high enough to oxidize the volatile matter and char of the coal particles. However, the wet recycle option results in a higher surface incident radiation on walls, especially on the rear wall as shown in Fig. 8. As a result, the heat transferred by radiation through walls increases from 269.60 MW in Case II to 280.09 MW in Case IV, which further results in the increase of the total transferred heat, from 302.11 MW in Case II to 311.53 MW in Case IV (see Table 5).

Our previous numerical study on the combustion test facility [17] showed that [O2]effective of ca. 30 vol% results in the closest match of the highest flame temperature and radiation heat transfer between the oxy-coal combustion case and the air-coal combustion case. However, it is 33 vol% in this study. This difference mainly attributes to the domain-based beam length, which is calculated as 3.6 (Vdomain/Adomain) [21]. The domain-based beam lengths are about 0.7 m and 9.5 m in the lab-scale boiler and large scale boiler respectively. The flue gas in the combustion test facility matches the approximation of optically thin gas more and its selfabsorption of flue gas is much smaller or negligible [31]. However, self-absorption cannot be neglected in the large scale combustion unit since more radiant energy is absorbed by the flue gas. Therefore, less flue gas amount, or a higher [O2]effective, is desired to reduce the self-absorption and match the air-coal combustion case in this study. 5.2. Load change of heat exchangers The above sections discussed the flame profile and heat transfer in the radiation zone of boiler, where mainly the evaporator (waterwall) locates. However, for a specific boiler, the proportion of the heat required by preheating, evaporating, and overheating of recycling water/steam is determined by the steam parameters [26]. If the transferred heat is changed in one of economizer, evaporator or superheater, it will change the heat transfer in other heat

Fig. 8. The predicted surface incident radiation contours for dry and wet recycle options in oxy-coal combustion (From left to right: front wall, left wall, rear wall, right wall).

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Air preheater 16 MW (2.0 %)

Economizer 271 MW (34.0 %) Air preheater 83 MW (10.4 %)

Evaporator 297 MW (37.2 %) Superheater 80 MW (10.0 %)

(Air case) Fuel input 798 MW (100 %)

Economizer 316 MW (39.6 %) Evaporator 276 MW (34.6 %) Superheater 84 MW (10.5 %)

(Oxy-29% case) Fuel input 798 MW (100 %)

Heat in the exhaust 67 MW (8.4 %)

Heat in the exhaust 106 MW (13.3 %) Flue gas recycle 223 MW

Economizer 275 MW (34.5 %) Air preheater 16 MW (2.0 %)

Evaporator 302 MW (37.8 %) Superheater 81 MW (10.2 %)

(Oxy-33% case) Fuel input 798 MW (100 %)

Heat in the exhaust 124 MW (15.5 %)

Economizer 230 MW (28.8 %) Evaporator

Air preheater 16 MW (2.0 %)

328 MW (41.1 %) Superheater 78 MW (9.8 %)

(Oxy-37% case) Fuel input 798 MW (100 %)

Heat in the exhaust 146 MW (18.3 %) Flue gas recycle 223 MW

Flue gas recycle 223 MW

Fig. 9. Sankey diagram from the specific case.

exchangers and further affect the performance of the whole plant. In order to understand the effect of oxy-coal combustion on the loads of heat exchangers, we also built a model in ASPEN PLUS [32] including the combustion reactor, evaporator, superheater, and economizer to study the energy balance. It was assumed that steam parameters of the steam cycle are 140 bar and 813 K and the minimum temperature difference in heat exchangers is 10 K. Results are illustrated by a Sankey diagram (Fig. 9). Comparing the reference case with Case II, they have similar loads of the economizer, evaporator, and superheater as expected. However, the loads are quite different in the air preheater. This is due to less flue gas passes through the air preheater in Case II. As a result, more heat will be contained in the flue gas in Case II from the view point of energy balance. The air preheater needs to be redesigned or re-evaluated. Comparing amongst the three oxy-coal combustion cases (Cases I, II, and III), the loads of the economizer and evaporator decrease when the [O2]effective increases, while the load of the superheater behaves in an opposite manner. This can be ascribed to the enhanced heat transfer in the radiation zone. Moreover, the operating conditions do not have many effects on the load of the superheater, because the heat required by overheating is determined by the steam parameters. So the change of the load between preheater and evaporator will not significantly affect the load of superheater. Based on above discussing, when retrofitting the air-coal boiler to the oxy-coal boiler, the application of 33 vol% of [O2]effective could result in the minimal change. And the necessary retrofit will be mainly focused on air preheater and heat recovery from the flue gas. On the other hand, when building a brand new oxy-coal boiler, in order to rebalance the load of each heat exchanger in the boiler, the feed water temperature after economizer can be reduced or part of superheating surfaces can be moved into the radiation zone to replace part of the evaporators. 6. Conclusions In this paper, a CFD model was developed for a large-scale combustion boiler, based on the geometry and operating conditions of

a real pulverized coal fired boiler of Electricidade de Portugal (EDP). It was used to simulate both the air-coal combustion and oxy-coal combustion technologies, focusing on the flame profile and the radiation heat transfer. Based on the simulation results, we have the following conclusions:  The highest flame temperature and total heat transferred through boiler side walls in the oxy-coal combustion case around 33 vol% of effective O2 concentration ([O2]effective) mostly match those in the air-coal combustion case.  Wet recycle option (the flue gas is recycled to the boiler before condensing) has little impact on the flame temperature, but results in a higher surface incident radiation on walls.  When retrofitting the air-coal boiler to the oxy-coal boiler, 33 vol% of [O2]effective could result in the minimal change for the existing boiler; while when building a brand new oxy-coal boiler, if boiler operates under a higher [O2]effective, to rebalance the load of each heat exchanger in the boiler, the feed water temperature after economizer can be reduced or part of superheating surfaces can be moved into the radiation zone to replace part of the evaporators.

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Please cite this article in press as: Hu Y et al. Numerical investigation of heat transfer characteristics in utility boilers of oxy-coal combustion. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.03.038